1. Field of the Invention
The present invention generally relates to optical modules. More particularly, exemplary embodiments of the invention concern an optical module that includes EMI control structures configured to interact with each other by way of through openings defined in a PCB of the optical module.
2. Related Technology
Fiber optics are increasingly used for transmitting voice and data signals. As a transmission medium, light provides a number of advantages over traditional electrical communication techniques. For example, light signals allow for extremely high transmission rates and very high bandwidth capabilities. Also, light signals are resistant to electromagnetic interferences that would otherwise interfere with electrical signals. Light also provides a more secure signal because it doesn't allow portions of the signal to escape from the fiber optic cable as can occur with electrical signals in wire-based systems. Light also can be conducted over greater distances without the signal loss typically associated with electrical signals on copper wire.
While optical communications provide a number of advantages, the use of light as a transmission medium presents a number of implementation challenges. In particular, the data carried by a light signal must be converted to an electrical format when received by a device, such as a network switch. Conversely, when data is transmitted to the optical network, it must be converted from an electronic signal to a light signal. A number of protocols define the conversion of electrical signals to optical signals and transmission of those optical, including the ANSI Fibre Channel (FC) protocol. The FC protocol is typically implemented using a transceiver module at both ends of a fiber optic cable. Each transceiver module typically contains a laser transmitter circuit capable of converting electrical signals to optical signals, and an optical receiver capable of converting received optical signals back into electrical signals.
Typically, a transceiver module is electrically interfaced with a host device—such as a host computer, switching hub, network router, switch box, computer I/O and the like—via a compatible connection port. Moreover, in some applications it is desirable to miniaturize the physical size of the transceiver module to increase the port density, and therefore accommodate a higher number of network connections within a given physical space. In addition, in many applications, it is desirable for the module to be hot-pluggable, which permits the module to be inserted and removed from the host system without removing electrical power. To accomplish many of these objectives, international and industry standards have been adopted that define the physical size and shape of optical transceiver modules to insure compatibility between different manufacturers. For example, in 2000, a group of optical manufacturers developed a set of standards for optical transceiver modules called the Small Form-factor Pluggable (“SFP”) Transceiver Multi-Source Agreement (“MSA”), incorporated herein by reference. In addition to the details of the electrical interface, this standard defines the physical size and shape for the SFP transceiver modules, and the corresponding host port, so as to insure interoperability between different manufacturers' products. There have been several subsequent standards, and proposals for new standards, including the XFP MSA for 10 Gigabit per second modules using a serial electrical interface, that also define the form factors and connection standards for pluggable optoelectronic modules, such as the published draft version 0.92 (XFP MSA), incorporated herein by reference.
As optical transmission speed provided by electronic modules increases, additional problems arise. For example, electronic devices and components operating at high frequencies typically emit signals referred to as electromagnetic interference. This electromagnetic interference, referred to as “EMI”, is electrical noise in the form of an electromagnetic wave. The phenomenon is undesirable because EMI can interfere with the proper operation of other electrical components. Optical transceiver packages, especially those operating at high transmission speeds, are especially susceptible to emitting EMI. In particular, the physical configuration of existing transceiver modules does a poor job of containing EMI—especially as the generating speed of the module increases. For example, as is shown in FIGS. 7A through 8C, a transceiver module 8 typically includes a housing 5 that contains a printed circuit board 10 and associated electrical and optical components. However, the housing 5 does not completely enclose the printed circuit board 10. Instead, a portion of the printed circuit board 10 is formed as an edge connector 12. The edge connector 12 includes a number of high speed traces for communicating signals to and from the electrical contacts on the edge connector 12. In operation, the edge connector 12 is capable of electrically and physically interfacing with a corresponding host connector 702 that is positioned on a host board 700.
Thus, in order for the edge connector 12 to be exposed externally to the module, the module housing 5 must provide an opening, shown at 20 in FIG. 6B. Moreover, insofar as the housing 5 is typically constructed of a conductive material, the opening 20 typically provides a minimum clearance area (the diagonal dimension of which is represented as “X” in FIG. 6B), so as to not electrically interfere with the high speed traces on the edge connector portion of the board 10. Unfortunately, this opening 20 also allows for the emission of an unacceptable amount if EMI; the emission is especially problematic as transmission speeds increase.
Therefore, there is a need in the industry for a pluggable module, such as an optoelectronic transceiver module, that is configured so as to minimize the emission of EMI. Preferably, the module configuration could be used in environments having high frequency data signal transmissions. Moreover, the module configuration should not affect the data signal integrity or the speed capabilities of the module. In addition, the electronic module should be implemented in a manner that meets existing standard form factors. Preferably, the module should maintain the ability to properly dissipate heat from the components inside the module.